In recent years, printed wiring boards with circuit patterns formed on an insulator substrate such as a plastic substrate, a ceramic substrate, or a metal substrate coated with a plastic or the like have been developed, and the technique of creating electronic circuits by soldering electronic components like IC devices, semiconductor chips, resistors, and condensers on the circuit pattern has been widely used.
To bond the lead terminals of such electronic components to predetermined sites on the circuit pattern, in general, a thin layer of a solder is formed in advance on the surface of a conductive circuit on a board, a solder paste or flux is printed on it, predetermined electronic components are mounted at the decided positions, and the thin solder layer or the thin solder layer and solder paste are reflowed to establish solder connections.
Recently there has been a demand for solder circuit boards of finer pitch for miniaturizing electronic products. Many fine pitch components such as 0.3 mm pitch QFP (Quad Flat Package) LSIs and CSPs (Chip Size Packages), and 0.15 mm pitch FCs (Flip Chips) are mounted. Therefore, very fine and accurate solder circuit patterns that can accommodate such fine pitch are being demanded in solder circuit boards.
Methods such as plating, HAL (Hot Air Leveling), and printing with a paste of solder powder and then reflowing it are used to form the solder circuit made of a solder film on a printed wiring board. When producing solder circuits by the plating method, however, it is difficult to form a thick solder layer, whereas achieving a fine pitch pattern is difficult with the HAL method or the solder paste printing method.
A method of forming solder circuits not requiring the cumbersome operation of aligning the circuit pattern, etc, which comprises imparting tackiness to the surface of a conductive circuit on a printed wiring board through a reaction with a tackiness-imparting compound, attaching a solder powder to these tacky parts, and then heating the printed wiring board to melt the solder and form the solder circuit, has been disclosed (refer, for example, to JP-A HEI 07-7244).
The method of attaching a solder powder to a printed wiring board disclosed in JP-A HEI 07-7244 is a dry process. This method sometimes causes the attachment of solder powder by static electricity to parts where it is not required, scattering of the powder, etc. Therefore, a wet process in which the printed wiring board is immersed in a slurry containing solder powder, to attach the solder power to the tackiness-imparted area of the circuit, has been disclosed (refer, for example, to JP-A 2006-278650).
In the method disclosed in JP-A HEI 07-7244, metal in the circuit part of the printed wiring board is allowed to react with a substance such as a naphthotraizole derivative to form a metal complex on the metallic part, and the solder powder is attached with the help of the tackiness of this metal complex. The adhesive force of this metal complex diminishes with time, which makes it necessary to attach the solder powder to the tacky area promptly after imparting tackiness to the circuit part of the printed wiring board. Therefore, the throughput has to be matched between the process of imparting tackiness to the circuit part of the printed wiring board and the process of attaching the solder powder to the tacky area. Moreover, if the throughput of the solder powder-attaching process is reduced due to reasons like a problem with the equipment, tackiness-imparted boards accumulate and their tackiness gets diminished. When this happens, it becomes necessary to renew the tackiness of the circuit parts of such printed wiring boards.
The purpose of the present invention is to provide a method that enables the smooth production of conductive circuit boards by prolonging the retention of tackiness imparted to the circuit part of the printed wiring board, i.e., increasing a permissible waiting time between the process of imparting tackiness to the circuit part of the printed wiring board and the process of attaching solder powder to the tacky area, and thus solve the aforesaid problems.